Current keel implant devices, such as tibial components, are solid with high stiffness are commonly associated with diaphyseal load transfer or stress shielding. This invention provides a porous section for allowing bone ingrowth and a reduced stiffness keel device for more anatomical load transfer. In addition, the porous section allows easier removal of the implant during revision procedure.
The present invention also relates to a device having two different porous surfaces attached directly or indirectly to one another and a method for forming the same. The two porous surfaces may be separated by a solid or fully dense (non-porous) layer.
The present application is particularly directed toward a method of forming porous or partially porous metallic structures having different porosities for bone ingrowth and soft tissue ingrowth or attachment. Porous structures may also be formed for polymer attachment.
One method of producing the different porous structures uses rapid prototyping to produce low density three-dimensional structures. This is useful in applications where porous and partially porous metallic structures, and more particularly metal porous structures with interconnective porosity are advantageous for use. In addition, composite structures of metal and porous ceramics or porous polymer can be used.
Many structures, especially in the medical arts, require two different surfaces, each adapted for their own purposes. Along this line, a structure may have a first surface which needs to be porous for tissue ingrowth and a second surface which could have porosity adapted to be a bearing surface. Those structures can be produced by Selective Laser Melting (SLM). See for example U.S. Patent Publication No. 2007/0142914, the disclosure of which is incorporated herein by reference. The field of free-form fabrication has seen many important recent advances in the fabrication of articles directly from computer-controlled databases. These advances, many of which are in the field of rapid prototyping of articles such as prototype parts and mold dies, have greatly reduced the time and expense required to fabricate articles, particularly in contrast to conventional machining processes in which a block of material, such as a metal, is machined according to the engineering drawings. One example of a modern rapid prototyping technology is the selective laser sintering process practiced by systems available from 3D Systems, Valencia, Calif. According to this technology, articles are produced in a layer-wise fashion, from a laser-fusible powder that is dispensed one layer at a time. The powder is fused, remelted or sintered, by the application of laser energy that is directed in raster-scan fashion to portions of the powder layer corresponding to a cross-section of the article. After fusing of the powder on one particular layer, an additional layer of powder is dispensed, and the process repeated with fusion taking place between the current layer and the previously laid layers, until the article is complete. In a first step of such process, a CAD file of an acetabular cup component is loaded into the Magics software package as a single part. The file may then be divided into three separate solid volumes having a 1.1 mm thick outer layer—this layer will be used to create the 80% porous bone ingrowth surface; 0.1 mm thick intermediate layer—this layer will be a fully dense layer that supports the bone ingrowth surface; and 0.8 mm thick inner layer—this will be used to create an interlock surface for a polymer injection molding. The three layers, when completed, will comprise the metal insert of the acetabular cup. Further, the first surface or portion may include different layers having different gradients of porosity. For example, the first surface may include an outer region having a porosity of approximately 80%. Moving more inwardly normal, with regard to the first surface, the porosity may alter such that the porosity is increased or in a preferred embodiment, the porosity decreases even until the porosity is almost zero. Of course, the present invention contemplates a situation where the porosity changes from position to position depending on the requirements of the device.
Cementless bone implant technologies provide a variety of porous surfaces that allows for bone ingrowth into the implant. This ingrowth allows for a better transfer of mechanical loads to the surrounding bone tissues and decreases bone resorption due to stress shielding. As with all primary implants sometimes a revision surgery is necessary to remove the implant. With non-cemented implants osseointegration blurs the bore/implant interface. This allows for the potential to lose large quantities of bone stock, when the implant is removed under conventional means. Other disadvantages of the current removal techniques include increased metal debris at and around the surgical site, increased heat and increased time in the operating room (OR). The proposed system addresses these issues and provides a method to remove porous material from cementless implants.
The porous material and manufacturing methods of the present invention can be used for other applications. One of the current concerns with modern total knee arthroplasty is the issue of femoral bone resorption due to stress shielding. This is most commonly found in the distal/anterior region behind the patellar groove of the implant. As femoral components are most commonly manufactured from Cobalt Chromium or titanium alloy, they have a significantly higher modulus of elasticity than the bone. Additionally, most femoral components are substantially rigid due to the amount of solid material that encompasses the space between the articular surface and the surfaces that mate with the resected femur. Bone remodels in the presence of load and in the natural knee, the articular surfaces are loaded by the patella and tibia. The replacement of that bearing with a stiff metal component shields the bone from much of the load in the anterior and posterior regions, leading to a lack of remodeling and ultimately, resorption. By making the femoral component essentially “hollow” and filling the space with foam, it becomes a more flexible component, which is better able to transfer the loads from the articular surfaces to the resected bone. This allows for remodeling of the bone in all regions and prevents stress shielding.
Current literature suggests several surgical methods to remove cementless implants during revision surgeries. These methods include the use of an oscillating cutting system such as oscillating saws and instruments which apply blunt striking force. Both methods have yielded positive effects, but present obvious disadvantages when used with solid implants.
Oscillating cutting systems are used to quickly cut up to the bone/implant interface and remove enough bone to allow the implant to be pulled from the bone. However, if the implant is not completely free, this results in large amounts of bone stock loss when the implant is extracted. With the oscillating system, when the blade reaches the porous coating of the otherwise solid implant, the high frequency motion can cause porous material to be dispersed throughout the surgical site. This may lead to subsequent revisions due to host response from debris particles or degradation of articulating surfaces due to body wear caused by the debris. Furthermore, friction between the blade and the implant can cause heat generation, in which the implant will act as a heat sink. The distribution of heat throughout the implant can transfer to the bone and cause bone tissue necrosis.
Blunt striking with an osteotome is also an effective means for separating the anchoring mechanisms of an implant from the planar force bearing surfaces. This allows access to the smaller anchoring point, which requires smaller, more precise tools to extract, or the surgeon can choose to leave them in. However, without a counter force, the bone provides the only resistance to implant movement. As such, the force from blunt striking may cause inadvertent damage to the bone surrounding the implant.
As used herein when referring to bones or other parts of the body, the term “proximal” means close to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means toward the head. The term “anterior” means toward the front part or the face and the term “posterior” means toward the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body.